1. Field of the Invention
The present invention relates to a coupled line filter; and, more particularly, to a coupled line filter usable in high frequency band.
2. Description of Related Art
Very high frequency is drawing attention as a radio frequency band favorable for using broadband signals and processing data at high speed. Specifically, frequency bands over 60 GHz are preferred and studied in both domestic and overseas countries to develop components and systems therefore. Also, to minimize the size of components and reduce the costs, Low-Temperature Co-fired Ceramic (LTCC) technology for three-dimensional integration is applied thereto.
Meanwhile, one of the essential components for a wireless communication system is a filter for selecting signals within desired frequency band. The filter has been an obstacle to miniaturization and cost reduction of the wireless communication system. In the wireless communication system, a filter using a lumped element, a microstrip or strip line filter using a transmission line, a resonator filter, a waveguide filter, and a surface acoustic wave (SAW) filter are used.
Among the diverse filters, the resonator filter is mainly used for microwave band due to its good electrical performances. The resonator filter is formed of resonators and coupling elements between them, and it can have very low losses in the desired frequency band. Also, the structure of resonators should be able to provide a coupling amount between resonators with very wide utility range to acquire the target frequency bandwidth. However, the resonator filter with a phase of approximately 90° transmission lines is rarely used to get low insertion losses in mm wave region because the resonator filter has a low quality coefficient when the coefficient filter uses a transmission line between the top and bottom surfaces that are grounded.
To make the filter using a transmission line have high quality coefficient, the insertion loss characteristics of the transmission line should be excellent. For this reason, a filter using a waveguide surrounded by a conductive material is usually used instead of the transmission line type filter. In the LTCC technology, the filter having a waveguide is realized by surrounding a side surface with multiple vias instead of the conductive material.
The LTCC filter using a waveguide has a resonator form and a structure coupling resonators similar to a conventional waveguide filter. If there is any difference, a first one of the resonators is directly coupled with an input port through microstrip line and waveguides stacked in multiple layers are connected through slots in the LTCC filter. U.S. Patent Publication Nos. 2004-0041663 and 2007-0120628 disclose such LTCC filters using a waveguide. However, the disclosed technologies has small number of coupling between resonators and the coupling amount between input/output port and a resonator is very small, there is a limitation in realizing a filter having broadband characteristics.
Meanwhile, among coupled line filters used in microwave band is an inter-digital filter, which will be described in detail with reference to the accompanying drawing.
FIG. 1 illustrates a typical inter-digital filter.
Referring to FIG. 1, a general inter-digital filter is a kind of a band pass filter used in microwave band. The band pass filter has a form of a planar substrate and a plurality of line resonators 110, 120, 130, 140, and 150 are disposed between an input line and an output line. The line resonators 110, 120, 130, 140, and 150 are realized by a plurality of transmission lines of the same form. The line resonators 110, 120, 130, 140, and 150 are disposed with a predetermined space between them. In FIG. 1, the space between the line first resonator 110 and the second line resonator 120 is marked as g12 and the space between resonators is determined according to a designed bandwidth. The line resonators 110, 120, 130, 140, and 150 are grounded only on one side and the grounded side is alternate. For example, when first sides (which is the lower sides) of the odd line resonators 110, 130, and 150 are grounded, the second sides (which is the upper sides) of the even line resonators 120 and 140 are grounded.
The line resonators 110, 120, 130, 140, and 150 of the inter-digital filter should have an electrical length of 90° at the center frequency of a band desired by a user. Here, the line resonators 110, 120, 130, 140, and 150 having an electrical length of 90° at the center frequency signifies that each of the line resonators 110, 120, 130, 140, and 150 has a length of λ/4 at the center frequency, where λ denotes a wavelength. For example, at 1 GHz, 1λ is 300 mm. Thus, a length of a line resonator at 1 GHz should be 75 mm to have an electrical length of 90°. Since the higher the frequency is, the shorter the wavelength becomes, the length of the line resonator becomes short.
To sum up, since a wavelength at a high frequency is short, the line resonator has to become short. For instance, when the center frequency is 60 GHz, a length of a line resonator should be 1.25 mm (in the air) to have an electrical length of 90° in the free space. However, when the inter-digital filter of FIG. 1 is actually designed, that is when the line resonators are realized on a predetermined substrate, the length of the line resonators is not that long compared to its width. Also, when the resonators become short, the quality coefficient (Q) affecting the insertion loss of the inter-digital filter becomes low.
This problem can be solved by using line resonators having an electrical length of 270° at high frequency instead of using those having an electrical length of 90°. However, when a coupled line filter is formed using the line resonators having an electrical length of 270°, there is a problem of a pass band being formed in a low frequency band, which is not desired by a user.